1. Field of the Invention
The present invention relates to a sheet finisher mounted on or operatively connected to a printer, copier, facsimile apparatus or similar image forming apparatus for executing preselected processing with a sheet driven out of the image forming apparatus. More particularly, the present invention relates to a sheet finisher including punching means.
2. Description of the Background Art
A sheet finisher of the type including punching means has customarily been used with an image forming apparatus. The punching means punches sheets sequentially driven out of an image forming apparatus one by one and is therefore free from heavy punching loads. In addition, this type of punching means enhances productivity. However, a problem with such punching means is that if a sheet entered the sheet finisher is skewed, shifted in the horizontal direction or otherwise dislocated, then the hole of the sheet cannot be accurately aligned with the holes of successive sheets.
In order to solve the above problem, there has been proposed a system that corrects the skew of a sheet and, in addition, reads one edge of the sheet parallel to the direction of sheet conveyance and then corrects the shift of the sheet in accordance with the resulting information. More specifically, this system determines the ideal position of a sheet to enter the sheet finisher on the basis of width information sent from an image forming apparatus. The system then compares the ideal position and the actual position of the sheet represented by the output of sheet edge sensing means and corrects a gap between the two positions.
For example, Japanese Patent Laid-Open Publication No. 10-279170 discloses a punching device including sheet edge sensing means responsive to the edge of a sheet, which is being conveyed by conveying means, parallel to the direction of sheet conveyance. First drive means moves the sheet edge sensing means in a direction perpendicular to the direction of sheet conveyance. Second drive means moves punching means in the direction perpendicular to the direction of sheet conveyance in accordance with information output from said sheet edge sensing means and representative of the position of the edge of the sheet. More specifically, the position where the sheet edge sensing means starts moving for sensing the edge of a sheet is set at an edge position parallel to the direction of sheet conveyance in accordance with the sheet size. The punching means is moved in the direction perpendicular to the direction of sheet conveyance on the basis of the edge information, thereby punching consecutive sheets at the same position without lowering image forming speed.
The prior art punching device described above has the following problems left unsolved. The punching device causes punching means and sheet edge sensing means to move integrally with each other and then stops the sheet edge sensing means when it senses the edge of a sheet. Subsequently, the punching device moves the punching means by using the stop position of the sheet edge sensing means as a reference. That is, the reference position for punching is coincident with the edge of a sheet and therefore varies sheet by sheet. Consequently, the punching device needs sophisticated control and is difficult to punch a sheet at an accurate position. Moreover, in the case of a sheet of irregular size not recognized by an image forming apparatus, the punching device cannot see the width of the sheet. As a result, even when the sheet is conveyed in an ideal position, the punching means cannot be moved to the center of the sheet or punch it with accuracy because the ideal position to be compared with the actual edge position is not known.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 9-249348.
It is an object of the present invention to provide a sheet finisher with punching means capable of accurately punching a sheet with simple control.
It is another object of the present invention to provide a sheet finisher with punching means capable of accurately punching a sheet at the center even when the sheet is of irregular size not recognized by an image forming apparatus.
A sheet finisher of the present invention includes a conveying device for conveying a sheet and a punch unit for punching the sheet being conveyed by the conveying device. A sheet edge sensor senses the edge of the sheet, which is being conveyed by the conveying device, parallel to the direction of sheet conveyance. A drive source moves the sheet edge sensor in a direction perpendicular to the direction of sheet conveyance. Another drive source moves the punch unit in the direction perpendicular to the direction of sheet conveyance in accordance with information output from the sheet edge sensor. A controller controls the two drive sources by a preselected pulse ratio, thereby causing the punch unit to accurately punch the sheet.